Tissue products, such as facial tissues and toilet tissues, are relatively light-weight, low density papers that are undoubtedly recognized as a fundamental staple commodity. The prime source of fibers used in preparing these tissue products are wood pulp fibers having an average fiber length of from less than 1 millimeter (&lt;1 mm) to approximately 2 mm. Such fibers include chemical wood pulps, such as sulfite and sulfate-process wood pulps (i.e., Kraft), and mechanical wood pulps, such as ground wood, ThermoMechanical Pulp (TMP) and ChemiThermoMechanical Pulp (CTMP). Pulps derived from both deciduous (i.e., hardwood) and coniferous (i.e., softwood) trees are used as fiber sources, in addition to fibers derived from recycled paper. These prior art tissue products further comprise minor amounts of chemical functional agents that include wet strength and dry strength binders, retention aids, surfactants, size, chemical softeners and the like, and reportedly demonstrate a balance of properties including strength and softness. It is noted that some of these prior art woodbased tissue products have a high degree of dusting or linting. Moreover, the inherent degree of coarseness associated with this fiber source, coupled with the presence of residual processing agents, results in a tissue product that could act as an irritant to users.
The above-referenced prior art tissue products are made from sheets of paper prepared using conventional papermaking processes and techniques, which include the steps of forming a pulp or aqueous fibrous slurry, depositing the slurry on a foraminous surface, such as a Fourdrinier wire or the surface of a forming cylinder employed in a cylinder mold papermaking machine, removing water from the deposited slurry by, for example, gravity or vacuum-assisted drainage, followed by adhering the resulting semi-dry sheet to the surface of a Yankee dryer, completely removing the water from the semi-dry sheet by evaporation, removing the essentially dry sheet from the Yankee dryer, and winding the resulting sheet onto a reel.
Papermaking fibers that are used in these tissue products are prepared by liberating individual fibers from the wood pulp into an aqueous slurry using conventional pulping methods and by refining, if necessary, to reduce fiber length.
The tissue industry has long recognized and sought to accommodate a segment of the general public that have existing medical conditions, such as external physical disruptions or maladies (e.g., swollen tissue), or that have demonstrated a propensity toward hypersensitive reactions to existing tissue products. Attempts to accommodate these individuals include the offering of tissue products that are devoid of fragrances, preservatives and other non-essential components or ingredients that may aggravate existing conditions or that may promote or encourage an allergic or other physical reaction thereto. However, even these altered products, presumably due in part to their inherent degree of coarseness, continue to present problems to this segment of the general public.
Economic and environmental concerns have prompted a recent trend in the tissue paper industry to reduce the amount of wood pulps used in products such as facial and toilet tissues. Methods of achieving such a reduction include the replacement of wood pulp fibers with high yield fibers or with fibers which have been recycled. Another such method is disclosed in U.S. Pat. No. 5,611,890 to Vinson et al. and involves the replacement of wood pulp with a lower cost, readily available filling material such as kaolin clay and calcium carbonate. Unfortunately, these methods generally tend to adversely affect the softness or tactile impression of these products.
The pleasing tactile impression of cotton has long been recognized and the use of cotton fibers has been common in some parts of the paper industry for a number of years. However, extending the use of this fiber source to tissue products presents significant technical and manufacturing hurdles at each stage in the production process from the selection of raw cotton linters through the linters pulping process and the tissue manufacturing process.
Raw cotton linters obtained from the cotton seed are characterized by grades that vary considerably in length, foreign particle or dirt content and in the degree of remnant staple fiber and gin motes. For example, first cut linters, which are principally used in banknotes and high quality stationary and document paper, are long (i.e., &gt;10 mm) and contain staple fiber and gin mote remnants. As a result, significant refining and cleaning problems are presented when attempting to prepare a cotton linter pulp. Additional processing problems are presented when these linter pulps are used to prepare facial and toilet tissues. It has been observed by the present applicant that excessive "roping" of the fibers occurs when an aqueous fibrous slurry containing such raw cotton linters is passed through pumps and cleaners in a conventional tissue production process. It has also been observed that even if the fibers are subsequently shortened by refining and beating, in an attempt to improve the physical characteristics of the resulting sheet, the sheet is "pock-marked" with hard pills.
Second and third cut linters vary depending on the country of origin. For example, second cut linters from Asia or Europe are significantly longer than American second cut linters and has a tendency to have pills, but to a lesser degree than noted above. Asian and European third cut linters are shorter than American second cuts but have a higher dirt content.
The long nature of cotton linter fibers has been identified as contributing to observed plugging of disk refiners used in domestic pulp mills and tissue mills. In specific regard to double disk refiners used in domestic pulp mills, it has been observed that if the gap between the bars of the refiner is too narrow and too shallow to allow clear passage of the fiber, these gaps will plug with hard lumps of fiber resulting in a loss of refining capability and in defects in the pulp sheets in the form of hard nits, pills and strings.
Different grades of cotton linter pulps are commercially available. However, it is noted that no single grade of these commercially available pulps can be used to manufacture tissue products that demonstrate a balance of properties including softness or reduced coarseness and strength. In addition, the numerical representations of relevant fiber lengths quoted by pulp manufacturers are both inadequate and misleading. Moreover, it has been observed that a seemingly apparent equivalency between commercially available pulps does not guarantee the preparation of equivalent tissue products.
Two instruments (i.e., a Clark Classifier and a Bauer McNett Classifier) are used in the linter industry to quantify relative fiber length. Both instruments operate on the principle of collecting fibers from dilute slurries on screens of decreasing coarseness. For example, a Clark Classifier is usually equipped with screens of U.S. 14, 30, 50 and 100-mesh. An aqueous fibrous slurry is first passed through the 14-mesh screen. Long fibers are retained on the screen or in a reservoir upstream of the screen. This process is then repeated on the 30, 50 and 100-mesh screens. Each reservoir is then drained and the fibers collected and weighed. The weight percent of fibers retained on each screen and associated reservoir is a numerical presentation of relative fiber length. Generally, the percent retained on the 14-mesh screen is quoted in specifications for the various pulp grades.
As noted hereinabove, despite similar specifications, cotton linter pulps may have significantly different characteristics. By way of example, a grade sold as "refined first cut cotton linter pulp" will start with a relatively long fiber but will be heavily refined and cut before the pulp is dried. Grade specifications will include a fiber length of 45 to 55% on U.S. 14-mesh. This so-called "first cut based pulp" has a marked tendency to rope and form pills but yields a tissue product having good paper strength. In comparison, a grade sold as a filter pulp will start production with a relatively short fiber blend but will only be refined enough to form a sheet on the pulp dryer. Grade specifications will include a fiber length of 40 to 50%. This filter pulp grade forms few strings or pills but yields a tissue product having relatively poor paper strength.
In addition to the uncertainties presented by the inherent variety within each raw cotton linter grade and each commercial grade of cotton linter pulp, it is further noted that cotton fibers are known to entangle more readily than wood fibers, thereby presenting yet additional processing hurdles.
Conventional tissue mill equipment is designed to accommodate or process relatively clean wood pulp fibers having an average fiber length of from &lt;1 mm to approximately 2 mm. As a result, the use of cotton linter pulps in the preparation of tissue products using such conventional equipment is immediately problematic. The process areas where the use of cotton linter pulp is the least compatible with conventional tissue mill equipment is in the stock preparation, pulping, pumping, refining and screening process areas. It has been observed that the use of these pulps results in fairly immediate plugging of the equipment where the screens and refiner(s) employed therein cannot provide the degree of cleaning and heavy refining needed without plugging. It is noted that refiners used in conventional tissue mills are generally undersized in that they cannot supply the necessary horsepower to process such cotton linter pulps.
There is a need in the art for a cotton fiber-based product that meets the needs of individuals with existing medical problems and the preferences of average consumers. It is therefore an object of the present invention to overcome the limitations and drawbacks detailed above.
It is a more specific object of the present invention to provide a tissue paper product prepared from cotton linter fibers.
It is an even more specific object to provide a tissue paper product that demonstrates a balance of properties including softness or reduced coarseness and strength.
It is yet a more specific object to provide a tissue paper product that reduces or eliminates any adverse reactions resulting from the use of such a product by individuals having existing medical conditions or hypersensitivities.
It is another object of the present invention to provide a process for preparing cotton linter tissue products that serves to overcome the significant technical and manufacturing problems presented by this fiber source.
It is yet a further object to provide a process for preparing a cotton linter pulp that may be readily processed in domestic tissue mills.